Today, optical-based data storage systems are commercially competitive due to their high storage density, relatively low cost, and random access capability. Moreover, magneto-optical data storage systems offer the added flexibility of allowing an optical medium to be erased and new data written in place of the erased section. This feature grants a user the capability to reuse an optical medium many times over by erasing old data and substituting new data in place thereof.
Basically, magneto-optical recording operates in the following manner. Data is stored as a series of binary bits (i.e., 1's and 0's). A laser beam is focused onto an optical medium, usually by means of a lens assembly. Initially, the optical medium is perpendicularly magnetized. To write a "1", the laser beam is pulsed at a high power for a short duration. This raises the temperature of the optical medium to such a degree that an externally applied magnetic field reverses the direction of magnetization in the heated region. When the medium returns to its lower ambient temperature, the "domain" retains its reverse magnetization.
Data is read from the optical medium by sensing the polarization of the linearly polarized light reflected from a perpendicularly magnetized medium. Hence, the magnetization transitions of the domains stored on the media can be read by determining the direction of the plane of polarization of the reflected light. The same laser beam used to write the data can also be used to generate the reflected light, provided that its power is kept relatively low to minimize any increase in the temperature to the disk.
The domains are "erased" by using the laser to perform the same thermal process used to write the data, except that an oppositely directed external magnetic field is applied. Thereby, the domains revert back to their original magnetization.
As described above, magneto-optical write operations are temperature dependent. In the prior art, the laser beam was kept at a constant power corresponding to an optimum temperature for writing data onto the medium. However, one drawback with the prior art method is that typically, magneto-optical recording devices are operated subject to a wide temperature range. For instance, the device might be operated under very cold conditions (e.g., outdoors, in front of an air conditioner vent, etc.) or very hot conditions (e.g., in direct sunlight, surrounded by equipment which are hot, etc.). Indeed, the temperature of the magneto-optical recording device can vary substantially between when it is first initialized and after it has operated over a length of time.
Consequently, under low temperature conditions, the laser beam's fixed power might be insufficient to properly heat the medium. As a result, the data might not get written onto the medium. Or, if the data does get written, it might be hard to detect when an attempt is subsequently made to read the data. In other words, the data is written so weakly that a read operation might miss part of the data. Conversely, under high temperature conditions, the laser beam's fixed power might be excessive. In extreme circumstances, this might cause permanent damage to the media. Another problem is that excessive power produces larger heated areas. As the areas increase in size, they start to interfere with one another. Consequently, these data bits become hard to detect because of the interference. This problem can be eliminated by writing the data bits a certain distance apart to eliminate the interference between two adjacent bits. However, this implementation is highly disadvantageous because less data is stored within a given area. In other words, the capacity of the device is reduced.
One prior art method for addressing this temperature problem was to implement a probe to measure the temperature of the magneto-optical recording device. Based on its measured temperature, the laser beam's power is adjusted accordingly (i.e., increased power for cooler temperatures and decreased power for higher temperatures). However, this method does not solve all the problems. First, the probe only senses the ambient temperature of the magneto-optical recording device rather than the actual temperature of the active layer of the optical medium. If the temperature of the optical medium is significantly different from that of the magneto-optical recording device (e.g., insertion of a cold optical disk into a magneto-optical disk drive that has been running a long time), the result could be an incorrect laser power being applied. Second, the media sensitivity typically varies from one medium to the next. This is due to imperfect manufacturing processes which lead to large tolerances. For example, one particular optical disk can have a significantly higher/lower sensitivity than another optical disk, even though they were both manufactured in the same lot. Consequently, different optical disks might require different amounts of laser power for optimum writing performance. Requiring tighter sensitivity tolerances would prohibitively increase the costs of the optical disks.
Therefore, what is needed is a simple, efficient, and accurate method/apparatus which adjusts the power of the laser beam for optimum performance, depending on that particular medium and its temperature. It is also desirable to provide such a solution without the need for additional hardware elements (other than those already available in the magneto-optical recording system).